Scores of people were killed and dozens more wounded on Saturday in the worst violence in recent Egyptian history as police opened fire on supporters of deposed President Mohammed Morsi.

Morsi supporters and the government offered widely different counts of the dead. The Muslim Brotherhood, the secret organisation through which Morsi ascended to the presidency, claimed as many as 200 were dead, while the Ministry of Interior announced implausibly that police never fired a live round at the protesters, despite all evidence to the contrary.

Health ministry officials revised the death toll throughout the day, with it hitting 80 by early Sunday. At least 792 people were injured, the ministry said.

A brief visit to a field hospital - one of three treating casualties - showed the brutality of what had taken place.

A McClatchy reporter counted 27 dead laid out on the hospital's floor, and as she left, three more bodies arrived, adding to a frantic and horrific scene. At least three of the dead had been shot in the head, and the gaping wounds left the victims' brains exposed.

Over and over, hospital workers would move a body to the ground and search the pockets for an identification card. When they found one, they wrote the deceased's name on an arm.

They then tied the body's hands and toes together, to prevent them from flopping around as the corpse was moved.

Often the workers had put a white wrap around the head to cover the gunshot wounds. Piles of national identification cards and personal belongings, like bloodied shirts and pants, were piled up nearby.

The only movement was that of doctors who seemed to jump around the corpses, reaching for bandages and the plaster needed to prepare shrouds, where the deceased's name would be written again.

One man who'd been assigned to clean blood from the floor shuffled through the scene, armed with a mop and a bucket that appeared to be more blood than water. Over and over he went on the same spot near one head, as the blood kept pouring out.

Doctors said the injuries could only have come from professional marksmen. Ebtesan Zain, a gynecologist, said she came to help her fellow doctors only to discover she was not needed - everyone she encountered was dead.

"Those injuries had to be done by snipers. It couldn't be anything else," Zain said. "They were shooting directly in the head between the eyes and in the chest."

US Secretary of State John Kerry in a statement said he had spoken with Egyptian Vice President Mohamed ElBaradei, and expressed "our deep concern about the bloodshed and violence."

The statement did not blame the military for the violence - the Obama administration has declined to call Morsi's ouster a coup - but it said "Egyptian authorities have a moral and legal obligation to respect the right of peaceful assembly and freedom of expression," a reference to officials' calls for an end to the pro-Morsi sit-in that has filled the streets of the Rabaa district for the past month.

"It is essential that the security forces and the interim government respect the right of peaceful protest, including the ongoing sit-in demonstrations," the statement said.

Secretary of Defence Chuck Hagel expressed similar sentiments in a call with Egypt's current strongman, Gen. Abdel-Fattah el-Sissi, according to a tweet by Pentagon spokesman George Little.

Just how the violence began is uncertain. The official version from the minister of the interior, Mohammed Ibrahim, who oversees the country's security services, said pro-Morsi protesters were headed toward pro-military supporters and that the military simply wanted to stop them from the 6th of October bridge, a key thoroughfare near Cairo's iconic Tahrir Square.

The official account said security forces fired tear gas to deter the pro-Morsi crowd and that 14 policemen were injured when the demonstrators retaliated.

Morsi supporters rejected that account.

"First of all, the march was heading to Abassya and there were no pro-military protesters there," said Adham Hassanien, 31, a journalist who works in a media center set up by Morsi supporters, referring to an area in Cairo where the Ministry of Defense is located.

He also discounted that Morsi supporters had attacked the security forces in any way, noting that the Rabaa protest had been going on for nearly a month. "Why would we attack them now? We are the ones who are losing people," he said.